1st Ruling: One must believe that every religion other than the Deen of Islam is Kufr (infidelity) and that only Islam would be accepted in the Hereafter. Allah Y says:
(And whoever seeks a Deen other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers.) (3:85)
One must believe that all non-Muslims are Kaafir, no matter what religion they adhere to, and that they are misguided. Allah Y says:
(Is he then whose bosom Allah has opened for the acceptance of Islam, so that he possesses a light from his Rubb, like him who is groping in the darkness of disbelief? Woe, then to those whose hearts are hardened against the remembrance of Allah! They are in manifest error.) (39:22)
Muslims should apply what they can of the rules that are pertinent to non-Muslims.
They should firmly believe that if non-Muslims die upon Kufr, while the Message of Islam reached them in an appropriate manner such that the evidence has been established against them, they are certainly from the People of the Fire even if they are People of the Book. The proof of this is in the Hadeeth of Abu Hurairah t that the Prophet r said:
‘By the One in whose hands is my life, no one from this Ummah whether Jew or Christian who hears of me and does not believe in what I have been sent with, except that he is from the People of the Fire.’
(Muslim)
2nd Ruling: The incumbency of disowning and disavowing the disbelievers and their nations, and to show enmity to them both openly and privately without differentiating between them. The reason for this is that they have rejected the right of Allah which is to worship Him alone and not associating any partners to Him, and they have rejected the Message of our Prophet r. Allah Y says:
(There is a good model for you in Ibraheem and those with him, when they said to their people, ‘We have nothing to do with you and with that which you worship beside Allah. We disbelieve all that you believe. There has arisen enmity and hatred between us and you forever, until you believe in Allah alone.) (60:4)
Allah Y says:
(O you who believe! Take not those for friends who make a jest and sport of your religion from among those who were given the Book before you, and the disbelievers. And fear Allah if you are believers.)
(5:57)
Allah Y says:
(O you who believe! Take not the Jews and the Christians for friends. They are friends one to another. And whoso among you takes them for friends is indeed one of them. Verily, Allah guides not the unjust people.) (5:51)
Hating and showing enmity to them should be done proportionately according to their disbelief and enmity to Islam.
To disavow them means to be displeased with them and their nations and to be displeased with their residing on the falsehood. To hate them denotes hating them from a religious perspective or to love them without reason which would mean loving and being pleased with their disbelief or giving precedence to their love over the love of Allah. As for loving them for a worldly purpose or for a certain reason such as the love a person holds for his father or a husband for his wife or a colleague for his colleague who is well mannered, and the one who is kind for his kindness, there is no sin in this.
To show enmity to them means hating them and not taking them as friends by helping them against the Muslims whether physically, monetarily, verbally or otherwise.
3rd Ruling: It is prohibited to imitate any of them in their specific customs such as baptizing children in church, or celebrating their religious holidays, such as Christmas. It is also prohibited for a person to imitate them in their manner of dress, and appearance and holding festivals similar to their festivals such as birthdays, and wedding anniversaries.
Allah Y says:
(Say, O People of the Book! Exceed not the limits in the matter of your religion unjustly, nor follow the evil inclinations of a people who went astray before and caused many to go astray, and who have strayed away from the Right Path.) (5:77)
Allah Y also says:
(O you who believe! Be not like those who have disbelieved…) (3:156)
The Prophet r said:
‘Whoever imitates a people he is from them.
(Abu Dawood)